Conventional methods used to sample industrial effluent aerosols (i.e. gas and particulate mixtures) require the insertion of a probe assembly into an opening in the duct or stack in order to withdraw a sample of the effluent gas stream. The withdrawn sample flows through the probe, into a sample line, and into an enclosed sample collection apparatus.
The specific configuration of the sampling equipment is prescribed in the methods contained in the Code of Federal Regulations, Title 40, Part 60 (40 CFR 60) and varies depending upon the sampling objective and applicable regulations. However, the essential configuration generally directs the aerosol sample through (in series) the sample probe, a filter, a series of impingers charged with reagents (usually liquid), and a vacuum line which leads to a pump and a gas metering instrument. At the end of this equipment train, the filtered gas sample is exhausted to the atmosphere.
Sample constituents are captured on the filter and in the impinger reagents. Particulate adhering to the walls of the sample probe and the front portion of the filter holder are recovered by rinsing with solvent. Recovered samples are delivered to the laboratory for analysis.
In standard laboratory procedures, each of the recovered sample components [(1) the filter and captured particulate, (2) the impinger reagents-optional, (3) and the solvent rinse] is transferred to a tared glass weighing dish and desiccated or otherwise treated. A sample is weighed at intervals until a constant weight is measured. For EPA Method 5, for example, a particulate filter sample is desiccated for 24 hours or oven dried. The sample is then weighed until a difference no greater than 0.5 mg or 1% of the sample weight (i.e., total weight less the tare weight) is measured between two weighings. The final sample weight is reported to the nearest 0.1 mg. It should be noted that small, below 20 milligrams, effluent particulate samples are common due to the excellent emissions control equipment now in use at many facilities and stringent emissions regulations. The sample and container are set aside in short term storage for several weeks until final reports have been received by the client. Samples to be stored for longer terms (sometimes 2 years or more) are transferred to selected plastic bags.
Within these standard procedures, some amount of sample weight measurement error is associated with the weights of the container (beaker) and sample. The weight of a typical 250 ml glass beaker (about 80-100 grams) is 4,000 to 5,000 times the weight of a 20-milligram particulate sample. Attempts to establish the particulate sample weight at .+-.10% accuracy must also be concerned with obtaining an accurate measurement of the beaker containing the sample. Even small errors in measurement of the total weight or beaker tare weight result in proportionally large variations relative to the contained 20-milligram sample. Measurement of the sample weight to .+-.10% would therefore be undermined. This significant burden upon accurate sample measurement is increased by unpredictable fluctuations in beaker weight resulting from activation of the beaker's glass surface.
Weight measurement error caused by contamination and activation of glass beakers is difficult to quantify. When beakers are cleaned for re-use, their surfaces are scraped and washed to remove sample materials. Sites on the large glass surface of the beaker become activated by reactions with cleaning solutions, mechanical cleaning, and samples. The adsorption, reaction, and desorption which occurs on these activated sites cause variable reactions with gases and particles in the laboratory atmosphere in unidentifiable and unpredictable ways. As a result, beaker weights can fluctuate, interfering with accurate or consistent measurement of the contained sample.